Electrically conductive noble metal-coated metallic particles, especially powders, are an important additive in the preparation of electrically conductive plastics, adhesives and inks, and in resin matrix based electromagnetic interference shielding materials.
The most commercially useful of such coated particles and powders are those wherein copper, nickel or aluminum substrates are coated with silver or gold. A number of processes have been developed over the years for the preparation of such noble metal-coated metallic materials.
For example, U.S. Pat. No. 3,202,488 to Ehrreich et al for "Silver-Plated Copper Powder" discloses a process for preparing silver-plated copper powder by replacement plating silver from silver cyanide solution whereby copper ions on the surface of the copper powder are replaced with silver ions from the solution.
U.S. Pat. No. 2,771,380 to Coleman et al for "Method of Plating Copper Particles With Silver" discloses a process for silver-plating copper particles requiring that the copper particles first be dry-mixed with an agent which maintains the copper particles in a separated or dispersed condition, prior to immersion in an aqueous silver plating bath.
U.S. Pat. No. 4,450,188 to Kawasumi for "Process for the Preparation of Precious Metal Coated Particles" discloses processes for coating a metal core material with a precious metal wherein a suspension of precious metal salt particles and dissolved precious metal salt ions; or a solution of dissolved precious metal salt ions; or a mixture of precious metal ions and a chelate of a precious metal compound in a suspended phase, are alternatively mixed with an aqueous suspension of core material particles, to carry out the coating of the core with the precious metal in a gelling suspension.
U.S. Pat. No. 4,652,465 to Koto et al. for "Process for the Production of a Silver Coated Copper Powder and Conductive Coating Composition" discloses a process wherein silver is precipitated on the surface of a copper powder by means of a silver complex solution containing a silver salt, an ammonium carbonate compound and ammonia water, which is added dropwise to a suspension of copper powder, alternatively, in water, in ammonia water, and in an aqueous solution of an ammonium carbonate compound.
U.S. Pat. No. 4,716,081 to Ehrreich for "Conductive Compositions and Conductive Powders for Use Therein" discloses a process for producing silver-coated non-noble metal powders, principally copper, by replacement plating from a solution containing ions of the noble metal, essentially as disclosed in U.S. Pat. No. 3,202,488, but further requiring high temperature heat treatment of the coated material at a temperature of 200.degree. C. for from 24 to several hundred hours or 150.degree. C. from 70 to 1500 hours.
U.S. Pat. No. 4,434,541 to Powers, Jr. for "Electromagnetic Shielding" discloses a process for preparing electromagnetic interference shielding materials utilizing electrically conductive solid metal particles consisting of an aluminum core on which it is first required to coat a layer of tin, zinc or nickel prior to plating with an outer coating of silver.
U.S. Pat. No. 3,989,606 to Kampert for "Metal Plating On Aluminum" discloses a process in which an aluminum substrate is first immersion coated with zinc prior to being electroplated with nickel.
All of the above processes, however, have certain disadvantages, which may result in the coated products produced not being of uniformly and consistently high quality, or the processes require some step, such as a long duration high temperature heat treatment in order to produce acceptable product, but which renders the process impractical and uneconomical for large scale commercial use. Some of the above processes have the disadvantage of requiring that the substrate material first be plated with an intermediate metal prior to coating with the precious metal. One utilizes a combination of immersion coating to produce the intermediate layer, followed by electro-plating to remove the intermediate layer and replace it with the outer coating of precious metal. Such a dual process has the disadvantage of also requiring a source of electricity, and depending on the costs of electricity, can be prohibitively costly in terms of both capital equipment costs and operating costs. Regardless of whether the precious metal coating is deposited by an immersion coating or an electro-plating process, in either case, the outer coating of precious metal may not completely coat or replace the intermediate layer, particularly because the coating with precious metal is performed in a single step, and may not be of uniform thickness, thereby affecting the physical and electrical properties of the final coated product, such as its corrosion resistance and electrical conductivity. In the past, it has sometimes occurred that producers of the coated materials have had to recoat the product after rejecting it for not having passed their own in-house quality control tests, or more embarrassingly, after rejection by their customers as being off specification and unacceptable for the intended end use. Both situations are costly to the producer, either in an economic sense or from the perspective of negatively affecting their business reputation.
Other earlier processes have the disadvantage of requiring the formation of suspensions or chelates of the precious metal ions, or suspensions of the substrate material, or both, and effect the coating reaction by a complex and messy gel-forming reaction. Still others have the disadvantage of requiring the addition of special additives to the substrate or to the plating solution bath in order to achieve a more acceptable quality of coated product.
The single greatest disadvantage of all of the earlier processes, however, has been the fact that they have been based on a single coating step in which the total amount of noble metal to be deposited is provided in one plating solution bath. Such processes present difficulties with respect to their capability of consistently producing uniformly coated product of high quality.
When the entire coating is effected in a single step, there is a tendency for uneven coating of all the substrate particles to occur. Some particles of the non-noble metal substrate can become coated with more than the desired amount of noble metal, while other particles of the substrate may be only partially coated or even completely uncoated. The latter is especially true when the substrate is a fine powder, having a large surface area.
Some of the parameters that play a major role in affecting the extent of coating of the substrate particles include the concentration of the noble metal ions in the plating solution bath; the size of the substrate particles; the homogeneity of the mixing and distribution of the substrate particles in the plating solution bath; the cleanliness and state of activation of the substrate material; and the efficiency of mixing and degree of contact between substrate particles and noble metal ions in the plating solution bath.
Where the substrate is a fine powder, local cohesive forces between powder particles may be sufficiently strong that they cannot be overcome when in the plating solution bath, causing clumping of the substrate particles. These clumps may remain even after stirring of the particles in the bath. When such clumps form, the outer surface of the particles to the center of a clump remains shielded against plating by the noble metal ions. Some have attempted to overcome this problem by introducing dispersing agents with the substrate material, however, this alone does not completely overcome the problem, and, in fact, may create other problems by introducing other chemical compounds into the plating solution baths. Care must be taken that the dispersing agent itself is chemically unreactive with respect to the precious metal and that it does not interfere with the coating process.
When coating is performed as a single step, there is also a tendency for any impurities in the plating solution bath to co-deposit on the surface of the substrate, together with the noble metal ions. These impurities may then prevent the subsequent plating of noble metal ions if the noble metal ions have little affinity for the surface of the impurities in comparison to the clean activated surface of the substrate itself. In such cases, the surface of the final product is an essentially noble metal coating interspersed by impurities. Depending on the nature and extent of the impurities, this phenomenon can greatly affect the physical and electrical properties of the final coated product. If the amount of impurities on the surface is large and of a nature as to adversely affect the corrosion resistance and electrical conductivity of the material, the entire batch of coated product will be off specification and unusable.
For example, the surface impurities may act as local sites at which oxidation or corrosion of the material can begin to occur. The impurities can also adversely change the electrical conductivity of the coated material.
Alternatively, impurities in the plating solution bath may first deposit on the substrate surface and subsequently become coated with noble metal, as long as the noble metal ions in the plating solution bath have sufficient affinity for coating the surface of the impurity. Where the bonding or surface adhesive forces between the substrate and the impurity or between the impurity and the noble metal which subsequently coats it are not as great as exists between the substrate and the noble metal itself, however, the coated product is susceptible to failure from several possible causes. The noble metal coating may abrade from the impurity leaving an exposed impurity or the noble metal-bearing impurity may become abraded from the substrate surface itself, leaving exposed substrate material. Depending on the nature of the impurity or the substrate material and the extent of the defect, either of these situations can have a significant effect on the properties of the coated product, possibly rendering it off-specification and unusable.
Degradation of materials containing such defects after incorporation in a finished product such as an electromagnetic shielding material is also more likely and can cause failure of the ultimate product. These defects can have a significant negative effect on the electrical conductivity of the material. Defects in the coated surface, either as impurities or exposed substrate, can themselves cause product failure by affecting the electrical properties of the coated material, or they can act as localized sites at which oxidation or corrosion may begin, ultimately leading to a change in the physical and electrical properties of the material and failure of the product in which the coated material has been incorporated. For example, exposed copper substrate is highly susceptible to corrosion if exposed to air or another oxygen-containing atmosphere.
Accordingly, it is an object of the present invention to teach a process that substantially eliminates all of the aforesaid problems inherent in previous processes requiring the formation of various suspensions or complexes, the formation of intermediate metal coating layers, the addition of special additives to promote the coating process, the use of combined immersion and electroplating techniques, or, generally, the use of only a single immersion coating step to effect coating of the precious metal, and which assures the consistent production of uniformly high quality coated product through the use of a multi-step coating process, with intermediate and final product rinsing steps.
The present invention is a significant improvement in and major contribution to the state of the art of preparing noble metal coated products in that it has been discovered as a result of extensive experimentation and testing that the aforesaid problems inherent in single plating step processes are overcome and high quality coated product of uniform consistency and long term stability is produced utilizing a coating process comprising a plurality of coating steps to plate-out the total desired amount of noble metal onto the substrate, with each of the individual plating steps being followed by a series of washing steps and a further series of washing steps being performed after the last step of the washing sequence following the final plating step.